(16)Aromatic Compounds: Comprehensive Notes

Organic Chemistry: Aromatic Compounds

Discovery of Benzene

• Benzene was first isolated in 1825 by Michael Faraday.
  • He determined the carbon to hydrogen ratio to be 1:1.
  • He named it "bicarburet of hydrogen."
• It was synthesized in 1834 by Eilhard Mitscherlich.
  • He determined the molecular formula to be C6H6.
  • He named it benzin.
• Early researchers classified these compounds as aromatic due to their pleasant smell and low C:H ratios.

Kekulé Structure

• Proposed in 1866 by Friedrich Kekulé shortly after the concept of multiple bonds was introduced.
• Failed to explain the existence of only one isomer of 1,2-dichlorobenzene.

Resonance Structures of Benzene

• Benzene is a resonance hybrid between two Kekulé structures.
• The carbon-carbon bond lengths in benzene are shorter than typical single-bond lengths but longer than typical double-bond lengths, giving it a bond order of 1.5.
• Resonance can be represented by drawing a circle inside the six-membered ring.

Structure of Benzene

• Each carbon atom is sp^2 hybridized with an unhybridized p orbital perpendicular to the ring.
• These p orbitals overlap around the ring, resulting in six pi electrons delocalized over the six carbon atoms.

Unusual Addition of Bromine to Benzene

• Bromine addition to benzene requires a catalyst like FeBr_3.
• The reaction results in the substitution of a hydrogen atom by a bromine atom.
• Direct addition of Br_2 to the double bond is not observed.

Resonance Energy

• Benzene's heat of hydrogenation is less negative than predicted.
• Predicted heat of hydrogenation: -359 kJ/mol.
• Observed heat of hydrogenation: -208 kJ/mol.
• The difference ( 151 kJ/mol ) is the resonance energy.

Molar Heats of Hydrogenation

• Cyclohexene: -120 kJ/mol
• Cyclohexadiene: -232 kJ/mol (-240 kJ/mol predicted)
• Benzene: -208 kJ/mol (-359 kJ/mol predicted) with a resonance energy of 151 kJ/mol
  • The resonance energy > 8 kJ/mol

Annulenes

• Annulenes are cyclic hydrocarbons with alternating single and double bonds.
• Benzene is a six-membered annulene, named [6]-annulene.
  • Cyclobutadiene is [4]-annulene.
  • Cyclooctatetraene is [8]-annulene.

Failures of the Resonance Picture

• Early theory suggested all cyclic conjugated hydrocarbons are aromatic.
• Cyclobutadiene is highly reactive and dimerizes before isolation.
• Cyclooctatetraene readily adds Br_2 to its double bonds.

MO Rules for Benzene

• Six overlapping p orbitals form six molecular orbitals (MOs).
  • Three are bonding, and three are antibonding.
• The lowest-energy MO has all bonding interactions and no nodes.
• As energy increases, the number of nodes increases.

MOs for Benzene

• \pi_1: All bonding
• \pi2 and \pi3: Bonding with one node each
• \pi4* and \pi5*: Antibonding with two nodes each
• \pi_6*: All antibonding

First MO of Benzene

• The first MO of benzene is entirely bonding with no nodes.
• It has very low energy due to six bonding interactions and electron delocalization.

Intermediate MO of Benzene

• Intermediate levels are degenerate (equal in energy).
• \pi2 and \pi3 each have one nodal plane.

All-Antibonding MOs of Benzene

• The all-antibonding \pi_6* has three nodal planes.
• Each adjacent p orbital pair is out of phase and interacts destructively.

Energy Diagram for Benzene

• Six p electrons fill the three bonding pi orbitals.
• All bonding orbitals are filled ("closed shell"), creating an extremely stable arrangement.

MOs for Cyclobutadiene

• \pi_1: All bonding
• \pi2 and \pi3: One bonding, one antibonding
• \pi_4*: All antibonding

Electronic Energy Diagram for Cyclobutadiene

• Following Hund's rule, two electrons occupy separate, nonbonding molecular orbitals.
• This diradical is highly reactive.

Polygon Rule

• The energy diagram for an annulene has the same shape as the cyclic compound with one vertex at the bottom.

Aromatic Requirements

• Cyclic structure with conjugated pi bonds.
• Each ring atom must have an unhybridized p orbital (sp^2 or sp).
• Continuous p orbital overlap around the ring.
• Planar structure for effective overlap.
• Delocalization of pi electrons must lower electronic energy.

Nonaromatic Compounds

• Lack a continuous ring of overlapping p orbitals.
• May be nonplanar.

Antiaromatic Compounds

• Cyclic and conjugated with overlapping p orbitals, but electron delocalization increases electronic energy.

Hückel’s Rule

• For a cyclic molecule with continuous overlapping p orbitals:
  • Aromatic if the number of pi electrons is (4N + 2), where N is an integer.
  • Antiaromatic if the number of pi electrons is (4N), where N is an integer.

Orbital Overlap of Cyclooctatetraene

• Cyclooctatetraene assumes a nonplanar tub conformation, avoiding overlap between adjacent pi bonds.
• Hückel's rule does not apply.

Annulenes

• [4]Annulene (cyclobutadiene) is antiaromatic.
• [8]Annulene (cyclooctatetraene) would be antiaromatic, but it's nonplanar, so it's nonaromatic.
• [10]Annulene is aromatic except for nonplanar isomers.
• Larger 4N annulenes are not antiaromatic due to flexibility and nonplanarity.

[10]Annulene

• All-cis [10]annulene has excessive angle strain in a planar conformation.
• The isomer with two trans double bonds cannot be planar due to hydrogen atom interference.

MO Derivation of Hückel’s Rule

• Aromatic compounds have (4N + 2) electrons and filled orbitals.
• Antiaromatic compounds have 4N electrons and unpaired electrons in two degenerate orbitals.

Cyclopentadienyl Ions

• Cation: An empty p orbital and four pi electrons, so it is antiaromatic.
• Anion: A nonbonding pair of electrons in a p orbital for six pi electrons, so it is aromatic.

Deprotonation of Cyclopentadiene

• Deprotonation of the sp^3 carbon creates a pair of electrons in one of the sp^3 orbitals.
• This sp^3 orbital can rehybridize to a p orbital.
• The six electrons in the p orbitals delocalize over all five carbon atoms, making the compound aromatic.

Orbital View of the Deprotonation of Cyclopentadiene

• Deprotonation allows overlap of all p orbitals.
• Cyclopentadiene is less stable than benzene and reacts readily with electrophiles.

Cyclopentadienyl Cation

• Hückel’s rule predicts that the cyclopentadienyl cation, with four pi electrons, is antiaromatic.
• Consistent with this, the cyclopentadienyl cation is not easily formed.

Cycloheptatrienyl Cation

• The cycloheptatrienyl cation has six pi electrons and an empty p orbital.
• It's easily formed by treating the corresponding alcohol with dilute (0.01N) aqueous sulfuric acid.
• Commonly known as the tropylium ion.

Cyclooctatetraene Dianion

• Cyclooctatetraene reacts with potassium metal to form an aromatic dianion.
• The dianion has ten pi electrons and is aromatic.

Pyridine Pi System

• Pyridine has six delocalized electrons in its pi system.
• Two nonbonding electrons on nitrogen are in an sp^2 orbital and do not interact with the pi electrons of the ring.

Pyridine

• Pyridine is basic, with a pair of nonbonding electrons available to abstract a proton.
• The protonated pyridine (the pyridinium ion) is still aromatic.

Pyrrole Pi System

• The pyrrole nitrogen atom is sp^2 hybridized with a lone pair of electrons in the p orbital.
• This p orbital overlaps with the p orbitals of the carbon atoms to form a continuous ring.
• Pyrrole is aromatic because it has six pi electrons (N = 1).

Pyrrole

• Pyrrole is aromatic because the lone pair on nitrogen is delocalized.
• N-protonated pyrrole is nonaromatic because the nitrogen is sp^3 hybridized.

Basic or Nonbasic?

• Pyrimidine has two basic nitrogens.
• Imidazole has one basic nitrogen and one nonbasic.
• Only one of purine’s nitrogens is basic.

Other Heterocyclics

• Cyclopentadienyl anion (six pi electrons)
• Pyrrole (six pi electrons)
• Furan (six pi electrons)
• Thiophene (six pi electrons)

Polynuclear Aromatic Hydrocarbons

• Naphthalene
• Anthracene
• Phenanthrene

Naphthalene

• Fused rings share two atoms and the bond between them.
• Naphthalene is the simplest fused aromatic hydrocarbon.

Larger Polynuclear Aromatic Hydrocarbons

• Formed in combustion (tobacco smoke).
• Many are carcinogenic.
• Epoxides form and combine with DNA bases leading to mutations.
• Examples: benzo[a]pyrene.

Allotropes of Carbon

• Amorphous: Small particles of graphite; charcoal, soot, coal, carbon black
• Diamond: A lattice of tetrahedral carbon atoms
• Graphite: Layers of fused aromatic rings

Diamond

• One giant molecule
• Tetrahedral carbon.
• Sigma bonds, 1.54 Å
• Electrical insulator

Graphite

• Planar layered structure
• Layer of fused benzene rings, bonds: 1.415 Å
• Only van der Waals forces between layers
• Conducts electrical current parallel to layers

Some New Allotropes

• Fullerenes: Five- and six-membered rings arranged to form a “soccer ball” structure
• Nanotubes: Half of a C_{60} sphere fused to a cylinder of fused aromatic rings

Fused Heterocyclic Compounds

• Purine
• Indole
• Benzimidazole
• Quinoline
• Benzofuran
• Benzothiophene
• Examples from nature and drugs: L-tryptophan, benziodarone, LSD, quinine

Common Names of Benzene Derivatives

• Phenol (benzenol)
• Toluene (methylbenzene)
• Aniline (benzenamine)
• Anisole (methoxybenzene)
• Styrene (vinylbenzene)
• Acetophenone (methyl phenyl ketone)
• Benzaldehyde
• Benzoic acid

Disubstituted Benzenes

• Numbers identify the relationship between groups.
  • Ortho- (o-) is 1,2-disubstituted.
  • Meta- (m-) is 1,3-disubstituted.
  • Para- (p-) is 1,4-disubstituted.

Three or More Substituents

• Use the smallest possible numbers.
• The carbon with a functional group is number 1.

Common Names for Disubstituted Benzenes

• m-xylene (1,3-dimethylbenzene)
• Mesitylene (1,3,5-trimethylbenzene)
• o-toluic acid (2-methylbenzoic acid)
• p-cresol (4-methylphenol)

Phenyl and Benzyl

• Phenyl indicates the benzene ring attachment.
• The benzyl group has an additional carbon.

Physical Properties of Aromatic Compounds

• Melting points: More symmetrical than corresponding alkanes, pack better into crystals, giving higher melting points.
• Boiling points: Dependent on dipole moment, so ortho > meta > para for disubstituted benzenes.
• Density: More dense than nonaromatics, less dense than water.
• Solubility: Generally insoluble in water.

Physical Properties of Benzene Derivatives

• Examples provided with melting points, boiling points, and densities.

IR and NMR Spectroscopy

• C=C stretching absorption at 1600 cm^{-1}.
• sp^2 C—H stretch just above 3000 cm^{-1}.
• ^1H NMR at δ 7–δ 8 for H’s on aromatic ring
• ^{13}C NMR at δ 120–δ 150, similar to alkene carbons

Mass Spectrometry

• The benzylic position is prone to fragmentation, leading to a benzyl cation (m/z = 91), which can rearrange to the tropylium ion.

UV Spectroscopy

• Benzene has a moderate band at 204 nm and a benzenoid band at 254 nm.

Ultraviolet Spectra of Benzene and Some Derivatives

• Comparison of UV spectra for benzene, ethylbenzene, m-xylene, bromobenzene, and styrene.